1. Field of the Invention
The present invention relates to a method of treating waste water to convert nitrogen-containing organic compounds into harmless gases such as nitrogen gas, carbon dioxide and the like, and an apparatus for treating waste water using the method. In particular, the invention relates to a method for treating waste water exhausted from a semiconductor manufacturing plant after dissolving and washing a photoresist, and relates to an apparatus suitable for treating such a waste water.
2. Prior Art
Waste water exhausted from various chemical plants, semiconductor manufacturing plants and the like sometimes includes nitrogen-containing organic compounds. Conventionally, as a method of treating waste water, an incineration method, a biological treatment method utilizing a function of micro-organisms, a decomposition treatment method utilizing super-critical water, and the like have been proposed.
The incineration method is the one in which waste water including nitrogen-containing organic compounds and/or hydrocarbons are burned at a high temperature so as to be oxidized, decomposed and treated, and nitrogen oxides (NOx) are produced as a result. Exhausting nitrogen oxides to the atmosphere will cause acid rain, and thus it is necessary to prepare treatment facilities for converting NOx to harmless compounds. Therefore, treatment of waste water according to incineration methods has such a disadvantage in that treatment facilities are apt to be large and require high construction costs.
The biological treatment method utilizes micro-organisms for decomposing organic compounds, such as a nitrogen-containing organic compound in waste water into nitrogen, water and carbon dioxide. This method has an advantageous effect in that organic compounds can be treated under moderate conditions and converted into harmless nitrogen gas and the like through decomposition thereof. However, this method has a disadvantage in that efficiency of decomposition is low, and an apparatus used for performing such a method must be large in order to guarantee sufficient ability to treat waste water. Large treatment facilities for the biological treatment method cause higher costs for their construction, similar to treatment facilities for the incineration method. Furthermore, the biological treatment method requires post-treatment of excess sludge produced through decomposition of organic compounds. Thus, the biological treatment method has some problems.
The decomposition treatment method utilizing super-critical water is one which utilizes physical and/or chemical properties of water in a super-critical state. In order to render water super-critical, it is necessary to maintain water in a state of high temperature and high pressure. Thus, heat resistance and pressure tightness are strictly required for treatment facilities, and devices such as a high pressure pump and a high pressure compressor are needed in order to accomplish the super-critical state. Furthermore, when organic compounds included in waste water are low in concentration, it is necessary to concentrate waste water prior to a decomposition treatment utilizing super-critical water.
Recently, a method which comprises gasifying waste water to produce waste gas, and oxidizing and decomposing the waste gas with a catalyst is suggested as a method of treating waste water without requiring large facilities and high construction costs.
For example, Japanese Patent unexamined laid-Open No.83081/90 (hereinafter referred to as prior art xe2x80x9c1xe2x80x9d) discloses a method of concentrating waste water through evaporating water therein, then separating a non-volatile contaminant and a vapor from the concentrated waste water, and oxidizing the vapor with a catalyst in the presence of water vapor. In this method, evaporation is carried out at a temperature of about 82xc2x0 C. to about 170xc2x0 C., and silica, alumina or chromic oxide is used as a catalyst. Therefore, harmful gases such as NOx and SOx are produced by catalytic oxidation and facilities are required for post-treating oxidized products.
Furthermore, Japanese Patent unexamined laid-open No.75913/97 (hereinafter referred to as prior art xe2x80x9c2xe2x80x9d) discloses a method of treating photographic processing waste water. In this method, waste water is heated by microwaves and a residual solid material is separated and removed, and thereafter, ammonia, which is included in a vaporized gas, is converted into nitrogen gas and water. The nitrogen gas can be exhausted to the atmosphere as it is, and the treated water can be drained into a sewage system. With this method, harmful NOx and SOx are not produced as a resulting product, and thus it is not necessary to provide post-treatment facilities, for resulting products along with an apparatus for treating waste water. However, prior art xe2x80x9c2xe2x80x9d does not disclose a catalyst with which ammonia gas is decomposed into nitrogen gas and water vapor. Thus, the method disclosed in prior art xe2x80x9c2xe2x80x9d cannot be directly applied to waste water including nitrogen compounds other than ammonia, particularly, waste water exhausted from a semiconductor manufacturing plant after cleaning photoresist. This is because the waste water includes nitrogen-containing organic compounds such as tetramethylammonium hydroxide, which is difficult to be gasified by microwaves.
The present invention has been carried out in view of these circumstances, and it is an object of the invention to provide a method of treating waste water including nitrogen compounds other than ammonia, in particular, nitrogen-containing organic compounds to convert the nitrogen-containing organic compounds into harmless gases such as nitrogen gas and carbon dioxide, without performing a post-treatment. Another object of the present invention is to provide an apparatus for treating waste water using the inventive method of treating waste water.
One aspect of the present invention is a method of treating waste water which comprises: first, spraying waste water including a nitrogen-containing organic compound into a heated space to gasify substantially all of the waste water to waste gas; and second, oxidizing the waste gas by contacting it with a catalyst to convert the nitrogen-containing organic compound into nitrogen gas, carbon dioxide and water vapor.
Another aspect of the present invention is an apparatus for treating waste water which comprises: a device for gasifying waste water, which device includes a sprayer for spraying waste water; a gasifying area into which waste water is introduced, and in which area the waste water is gasified by the gasifying device to produce waste gas from the waste waster; and a reaction area which contains a catalyst for oxidizing waste gas delivered from the gasifying area, and from which reaction area an oxidized gas is exhausted.